This problem considers the movement of a benzene plume in a confined aquifer toward a lake used for recreation and drinking water supply. This concerns the local municipality - the long-term health effects of long-term exposure above the Maximum Contaminant Level (MCL) of 0.005 mg/L is known to cause anemia, decrease in blood platelets, and increased risk of cancer.

A. Calculate the specific discharge through the system. If the benzene travels with the water velocity, estimate how long it will take to contaminate the lake. Assume a homogeneous aquifer: K=10.7 m/d, n=0.15

B. A few boreholes were drilled to better understand the aquifer. The initial data suggests that the aquifer is not homogeneous; rather, it is composed of four layers (see fig. below). Discuss the similarities and/or differences between the two systems (consider the specific discharge and travel time).

C. The local municipality has insisted that even more borehole data are collected to better understand the risk to lake contamination.  A statistical analysis revealed the following information about the aquifer:

• Mean hydraulic conductivity 10.7 m/d
• LnK variance: 1.8
• Vertical correlation scale: 6m
• effective porosity: 0.15

Assess the risk, or probability of the contaminant concentration exceeding the MCL at the aquifer-lake interface. Use MAGNET to generate the stochastically stratified K field. You should calculate the risk for different times. Assume that t=0 coincides with the initial position of the plume at the monitoring well, as depicted in the graphic below. You may also assume that the horizontal scale is 100 times larger than the vertical scale (i.e., the aquifer is essentially "perfectly stratified").

MAGNET/Modeling Hints:

• Use ‘Synthetic mode’ in MAGNET to create a model domain.
• Add prescribed head line and zone features along the left and right edges to represent the head measured at the monitoring well and lake, respectively.
• Use a zone feature of the entire aquifer area to assign the hydraulic conductivity as a random field realization.
• Add a monitoring well at the aquifer-lake interface to track concentrations as the simulation proceeds.
• Analyze the plots and charts using the DisplayCharts tool ( 'Analysis Tools' > 'Analysis' > 'Display Charts' )